Mechanical properties of fasteners · EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM...

BRITISH STANDARD BS EN20898-2:1994ISO 898-2:1992

Mechanical properties of fasteners —

Part 2: Nuts with specified proof load values — Coarse thread

The European Standard EN 20898-2:1993 has the status of aBritish Standard

UDC 621.882.3

Copyright British Standards Institution Provided by IHS under license with BSI

Not for ResaleNo reproduction or networking permitted without license from IHS

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BS EN 20898-2:1994

This British Standard, having been prepared under the direction of the General Mechanical Engineering Standards Policy Committee, was published under the authority of the Standards Board and comes into effect on15 June 1994

© BSI 01-1999

First published April 1992Second edition June 1994

The following BSI references relate to the work on this standard:Committee reference GME/9Draft for comment 90/79942 DC

ISBN 0 580 23137 2

Cooperating organizations

The European Committee for Standardization (CEN), under whose supervision this European Standard was prepared, comprises the national standards organizations of the following countries:

Austria Oesterreichisches Normungsinstitut

Belgium Institut belge de normalisation

Denmark Dansk Standardiseringsraad

Finland Suomen Standardisoimisliito, r.y.

France Association française de normalisation

Germany Deutsches Institut für Normung e.V.

Greece Hellenic Organization for Standardization

Iceland Technological Institute of Iceland

Ireland National Standards Authority of Ireland

Italy Ente Nazionale Italiano di Unificazione

Luxembourg Inspection du Travail et des Mines

Netherlands Nederlands Normalisatie-instituut

Norway Norges Standardiseringsforbund

Portugal Instituto Portuguès da Qualidade

Spain Asociación Española de Normalización y Certificación

Sweden Standardiseringskommissionen i Sverige

Switzerland Association suisse de normalisation

United Kingdom British Standards Institution

Amendments issued since publication

Amd. No. Date Comments



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BS EN 20898-2:1994

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Contents

PageCooperating organizations Inside front coverNational foreword ii

Foreword 21 Scope 32 Normative references 33 Designation system 44 Materials 45 Mechanical properties 46 Proof load values 87 Failure loads for nuts with nominal height $ 0.5 D but < 0.8 D 88 Test methods 89 Marking 9

Annex A (informative) Loadability of bolted connections 12Annex B (informative) Bibliography 16Annex ZA (normative) Normative references to international publications with their relevant European publications 17National annex NA (informative) Committees responsible Inside back coverNational annex NB (informative) Cross-references Inside back cover

Figure 1 — Axial tensile test 8Figure 2 — Axial compressive test 9Figure 3 — Examples of marking with designation symbol 9Figure 4 — Examples of marking with code symbol (clock-face system) 9Figure 5 — Left-hand thread marking 11Figure 6 — Alternative left-hand thread marking 11

Table 1 — Reduction in thread strength 3Table 2 — Designation system for nuts with nominal heights $ 0.8 D 4Table 3 — Designation system and stresses under proof load for nuts with nominal heights $ 0.5 D but < 0.8 D 4Table 4 — Limits of chemical composition 4Table 5 — Mechanical properties 5Table 6 — Proof load values — Coarse thread 7Table 7 — Minimum bolt stress when stripping occurs 8Table 8 — Marking symbols for nuts with property classes in accordance with 3.1 10Table 9 — Marking for nuts with property classes in accordance with 3.2 10Table A.1 — Property classes for bolts and screws 15Table A.2 — Effective maximum hardness within the thread engagement part of the bolt 15Table A.3 — Heights of hexagon nuts 15



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BS EN 20898-2:1994

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National foreword

This British Standard has been prepared under the direction of the General Mechanical Engineering Standards Policy Committee and is the English language version of EN 20898-2:1993 Mechanical properties of fasteners — Part 2: Nuts with specified proof load values — Coarse thread, published by the European Committee for Electrotechnical Standardization (CEN). It supersedes BS EN 20898-2:1992 which is withdrawn. EN 20898-2:1993 was produced as a result of international discussion in which the UK took an active part. It is one of a series of standards that are under preparation based on ISO 898.This Part is identical with ISO 898-2:1992 published by the International Organization for Standardization (ISO).A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application.

Compliance with a British Standard does not of itself confer immunity from legal obligations.

Summary of pagesThis document comprises a front cover, an inside front cover, pages i and ii, the EN title page, pages 2 to 18, an inside back cover and a back cover.This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.



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EUROPEAN STANDARD

NORME EUROPÉENNE

EUROPÄISCHE NORM

EN 20898-2

December 1993

UDC 621.882.3 Supersedes EN 20898-2:1991

Descriptors: Fasteners, nuts (fasteners), coarse threads, specifications, mechanical properties, tests, designation, marking

English version

Mechanical properties of fasteners — Part 2: Nuts with specified proof load values — Coarse thread

(ISO 898-2:1992)

Caractéristiques mécaniques des éléments de fixation — Partie 2: Ecrous avec charges d’épreuve spécifiées — Filetage à pas gros (ISO 898-2:1992)

Mechanische Eigenschaften von Verbindungselementen — Teil 2: Muttern mit festgelegten Prüfkräften — Regelgewinde (ISO 898-2:1992)

This European Standard was approved by CEN on 1993-12-16. CEN membersare bound to comply with the CEN/CENELEC Internal Regulations whichstipulate the conditions for giving this European Standard the status of anational standard without any alteration.Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the Central Secretariat or to anyCEN member.This European Standard exists in three official versions (English, French,German). A version in any other language made by translation under theresponsibility of a CEN member into its own language and notified to theCentral Secretariat has the same status as the official versions.CEN members are the national standards bodies of Austria, Belgium,Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy,Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland andUnited Kingdom.

CEN

European Committee for StandardizationComité Européen de NormalisationEuropäisches Komitee für Normung

Central Secretariat: rue de Stassart 36, B-1050 Brussels

© 1993 Copyright reserved to CEN members Ref. No. EN 20898-2:1993 E



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EN 20898-2:1993

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Foreword

This European Standard has been taken over by Technical Committee CEN/TC 185, Threaded and non-threaded mechanical fasteners and accessories, from the work of ISO/TC 2 of the International Organization for Standardization.The text was submitted to the Unique Acceptance Procedure (UAP) and approved as a European Standard.This European Standard supersedes EN 20898-2:1991.NOTE The International Standard ISO 6157-2 Surface discontinuities of nuts (see clause 8.3) is not yet available; in the interim reference shall be made to EN 493 Fasteners — Surface discontinuities — Nuts.

This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by June 1994, and conflicting national standards shall be withdrawn at the latest by June 1994.In accordance with the CEN/CENELEC Internal Regulations, the following countries are bound to implement this European Standard: Austria, Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.NOTE Normative references to international publications are listed in annex ZA (normative)



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EN 20898-2:1993

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1 ScopeThis International Standard specifies the mechanical properties of nuts with specified proof load values when tested at room temperature (see ISO 1). Properties will vary at higher and lower temperature.It applies to nuts

— with nominal thread diameters up to and including 39 mm;— of triangular ISO thread and with diameters and pitches according to ISO 68 and ISO 262 (coarse thread);— with diameter/pitch combinations according to ISO 261 (coarse thread);— with thread tolerances 6H according to ISO 965-1 and ISO 965-2;— with specific mechanical requirements;— with widths across flats as specified in ISO 272 or equivalent;— with nominal heights greater than or equal to 0,5D1);— made of carbon steel or low alloy steel.

It does not apply to nuts requiring special properties such as

— locking abilities (see ISO 2320);— weldability;— corrosion resistance (see ISO 3506);— ability to withstand temperatures above + 300 °C or below – 50 °C.

NOTE 1 Nuts made from free-cutting steel should not be used above + 250 °C.NOTE 2 For special products such as nuts for high-strength structural bolting, and overtapped nuts for use with hot-dipped galvanized bolts, see the product standards for appropriate valuesNOTE 3 For assemblies with threads having tolerances wider than 6H/6g, there is an increased risk of stripping; see also Table 1.NOTE 4 In the case of thread tolerances other or larger than 6H, a decrease of the stripping strength should be considered (see Table 1).

Table 1 — Reduction in thread strength

2 Normative referencesThe following standards contain provisions which, through reference in this text, constitute provisions of this part of ISO 898. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this part of ISO 898 are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.ISO 1:1975, Standard reference temperature for industrial length measurements.ISO 68:1973, ISO general purpose screw threads — Basic profile.ISO 261:1973, ISO general purpose metric screw threads — General plan.ISO 262:1973, ISO general purpose metric screw threads — Selected sizes for screws, bolts and nuts.ISO 272:1982, Fasteners — Hexagon products — Widths across flats.ISO 286-2:1988, ISO system of limits and fits — Part 2: Tables of standard tolerance grades and limit deviations for holes and shafts.ISO 724:1978, ISO metric screw threads — Basic dimensions.ISO 965-1:1980, ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data.ISO 965-2:1980, ISO general purpose metric screw threads — Tolerances — Part 2: Limits of sizes for general purpose bolt and nut threads — Medium quality.ISO 4964:1984, Steel — Hardness conversions.ISO 6157-2:—2), Fasteners — Surface discontinuities — Part 2: Nuts with threads M5 to M39.ISO 6506:1981, Metallic materials — Hardness test — Brinell test.ISO 6507-1:1982, Metallic materials — Hardness test — Vickers test — Part 1: HV 5 to HV 100.ISO 6508:1986, Metallic materials — Hardness test — Rockwell test (scales A – B – C – D – E – F – G – H – K).

1) D is the nominal diameter of the internal thread in accordance with ISO 724.

ThreadTest load, %

Thread tolerances

greater than

less than or equal to 6H 7H 6G

— M2,5 100 — 95,5

M2,5 M7 100 95,5 97

M7 M16 100 96 97,5

M16 M39 100 98 98,5

2) To be published.



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3 Designation system3.1 Nuts with nominal heights $ 0,8D (effective lengths of thread $ 0,6D)

Nuts with nominal heights $ 0,8D (effective lengths of thread $ 0,6D) are designated by a number to indicate the maximum appropriate property class of bolts with which they may be mated.Failure of threaded fasteners due to over-tightening can occur by bolt shank fracture or by stripping of the threads of the nut and/or bolt. Shank fracture is sudden and therefore easily noticed. Stripping is gradual and therefore difficult to detect and this introduces the danger of partly failed fasteners being left in assemblies.It would therefore be desirable to design threaded connections so that their mode of failure would always be by shank fracture but, unfortunately, because of the many variables which govern stripping strength (nut and bolt material strengths, thread clearances, across-flats dimensions, etc.), nuts would have to be objectionably thick to guarantee this mode in all cases.A bolt or screw of thread M5 to M39 assembled with a nut of the appropriate property class, in accordance with Table 2, is intended to provide an assembly capable of being tightened to the bolt proof load without thread stripping occurring.However, should tightening beyond bolt proof load take place, the nut design is intended to ensure at least 10 % of the over-tightened assemblies fail through bolt breakage in order to warn the user that the installation practice is not appropriate.NOTE 5 For more detailed information on the strength of screw thread assemblies, see annex A.

Table 2 — Designation system for nuts with nominal heights $ 0,8D

3.2 Nuts with nominal heights $ 0,5D but < 0,8D (effective heights of thread $ 0,4D but < 0,6D)

Nuts with nominal heights $ 0,5D but < 0,8D (effective height of thread $ 0,4D but < 0,6D) are designated by a combination of two numbers: the second indicates the nominal stress under proof load on a hardened test mandrel, while the first indicates that the loadability of a bolt-nut assembly is reduced in comparison with the loadability on a hardened test mandrel and also in comparison with a bolt-nut assembly described in 3.1. The effective loading capacity is not only determined by the hardness of the nut and the effective height of thread but also by the tensile strength of the bolt with which the nut is assembled. Table 3 gives the designation system and the stresses under proof load of the nuts. Proof loads are shown in Table 6. A guide for minimum expected stripping strengths of the joints when these nuts are assembled with bolts of various property classes is shown in Table 7.

Table 3 — Designation system and stresses under proof load for nuts with nominal

heights $ 0,5D but < 0,8D

4 MaterialsNuts shall be made of steel conforming to the chemical composition limits specified in Table 4.

Table 4 — Limits of chemical composition

Nuts of property classes 05, 8 (style 1 above M16), 10 and 12 shall be hardened and tempered.

5 Mechanical propertiesWhen tested by the methods described in clause 8, the nuts shall have the mechanical properties set out in Table 5.

Property class of

nut

Mating boltsNuts

Style 1 Style 2

Property class Thread range Thread ranges

4 3.6; 4.6; 4.8 > M16 > M16 —5 3.6; 4.6; 4.8 # M16

# M39 —5.6; 5.8 # M39

6 6.8 # M39 # M39 —

8 8.8 # M39 # M39> M16# M39

9 9.8 # M16 — # M1610 10.9 # M39 # M39 —12 12.9 # M39 # M16 # M39NOTE In general, nuts of a higher property class can replace nuts of a lower property class. This is advisable for a bolt/nut assembly going into a stress higher than the yield stress or the stress under proof load.

Property class of nut

Nominal stress under proof load

Actual stress under proof load

N/mm2 N/mm2

04 400 38005 500 500

Property class

Chemical composition limits(check analysis), %

C Mn P Smax. min. max. max.

4a; 5a; 6a — 0,50 — 0,060 0,1508;9 04a 0,58 0,25 0,060 0,15010b 05b 0,58 0,30 0,048 0,05812b — 0,58 0,45 0,048 0,058a Nuts of these property classes may be manufactured from free-cutting steel unless otherwise agreed between the purchaser and the manufacturer. In such cases, the following maximum sulfur, phosphorus and lead contents are permissible:

sulfur 0,34 %; phosphorus 0,11 %; lead 0,35 %.b Alloying elements may be added, if necessary, to develop the mechanical properties of the nuts.



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Table 5 — Mechanical properties

Thread

Property class

04 05 4

Stress under

proof load Sp

Vickers hardness HV Nut

Stress under proof

load Sp

Vickers hardness

HVNut

Stress under

proof load Sp


greater than

less than or equal to N/mm2 min. max. state style N/mm2 min. max. state style N/mm2 min. max. state style

— M4

M4 M7 — — — — —

M7 M10 380 188 302 NQTa thin 500 272 353 QTb thin

M10 M16

M16 M39 510 117 302 NQTa 1

Thread

Property class

5c 6 8

Stress under proof

load Sp

Vickers hardness

HVNut

Stress under proof

load Sp

Vickers hardness

HVNut

Stress under proof

load Sp


Stress under proof

load Sp

Vickers hardness

HVNut

greater than

less than or equal to N/mm2 min. max. state style N/mm2 min. max. state style N/mm2 min. max. state style N/mm2 min. max. state style

— M4 520

130

600

150

800 180

302 NQTa

1— — — — —

M4 M7 580 670 855

M7 M10 590 302 NQTa 1 680 302 NQTa 1 870 200

M10 M16 610 700 880

M16 M39 630 146 720 170 920 233 353 QTb 890 180 302 NQTa 2



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Table 5 — Mechanical properties

Thread

Property class

9 10 12

Stress under proof

load Sp

Vickers hardness

HVNut

Stress under proof

load Sp

Vickers hardness

HVNut

Stress under proof

load Sp

Vickers hardness

HVNut

Stress under proof

load Sp

Vickers hardness

HVNut

greater than

less than or equal to N/mm2 min. max. state style N/mm2 min. max. state style N/mm2 min. max. state style N/mm2 min. max. state style

— M4 900 170 1 040 1 140

295 353 QTb 1

1 150

M4 M7 915

188

1 040 1 140 1 150

M7 M10 940 302 NQTa 2 1 040 272 353 QTb 1 1 140 1 160 272 353 QTb 2

M10 M16 950 1 050 1 170 1 190

M16 M39 920 1 060 — — — — — 1 200NOTE Minimum hardness is mandatory only for heat-treated nuts and nuts too large to be proof-load tested. For all other nuts, minimum hardness is not mandatory but is provided for guidance only. For nuts which are not hardened and tempered, and which satisfy the proof-load test, minimum hardness shall not be cause for rejection.a NQT = Not quenched or tempered.

b QT = Quenched and tempered.

c The maximum bolt hardness of property classes 5.6 and 5.8 will be changed to be 220 HV in the next revision of ISO 898-1:1988. This is the maximum bolt hardness in the thread engagement area whereas only the thread end or the head may have a maximum hardness of 250 HV. Therefore the values of stress under proof load are based on a maximum bolt hardness of 220 HV.



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Table 6 — Proof load values — Coarse thread

ThreadThread pitch

Nominal stress area of the mandrel As

Property class

04 05 4 5 6 8 9 10 12

Proof load (As × Σp)

N

mm mm2 Style 1 Style 1 Style 1 Style 1 Style 2 Style 2 Style 1 Style 1 Style 2

M3 0,5 5,03 1 910 2 500 — 2 600 3 000 4 000 — 4 500 5 200 5 700 5 800

M3,5 0,6 6,78 2 580 3 400 — 3 550 4 050 5 400 — 6 100 7 050 7 700 7 800

M4 0,7 8,78 3 340 4 400 — 4 550 5 250 7 000 — 7 900 9 150 10 000 10 100

M5 0,8 14,2 5 400 7 100 — 8 250 9 500 12 140 — 13 000 14 800 16 200 16 300

M6 1 20,1 7 640 10 000 — 11 700 13 500 17 200 — 18 400 20 900 22 900 23 100

M7 1 28,9 11 000 14 500 — 16 800 19 400 24 700 — 26 400 30 100 32 900 33 200

M8 1,25 36,6 13 900 18 300 — 21 600 24 900 31 800 — 34 400 38 100 41 700 42 500

M10 1,5 58 22 000 29 000 — 34 200 39 400 50 500 — 54 500 60 300 66 100 67 300

M12 1,75 84,3 32 000 42 200 — 51 400 59 000 74 200 — 80 100 88 500 98 600 100 300

M14 2 115 43 700 57 500 — 70 200 80 500 101 200 — 109 300 120 800 134 600 136 900

M16 2 157 59 700 78 500 — 95 800 109 900 138 200 — 149 200 164 900 183 700 186 800

M18 2,5 192 73 000 96 000 97 900 121 000 138 200 176 600 170 900 176 600 203 500 — 230 400

M20 2,5 245 93 100 122 500 125 000 154 400 176 400 225 400 218 100 225 400 259 700 — 294 000

M22 2,5 303 115 100 151 500 154 500 190 900 218 200 278 800 269 700 278 800 321 200 — 363 600

M24 3 353 134 100 176 500 180 000 222 400 254 200 324 800 314 200 324 800 374 200 — 423 600

M27 3 459 174 400 229 500 234 100 289 200 330 500 422 300 408 500 422 300 486 500 — 550 800

M30 3,5 561 213 200 280 500 286 100 353 400 403 900 516 100 499 300 516 100 594 700 — 673 200

M33 3,5 694 263 700 347 000 353 900 437 200 499 700 638 500 617 700 638 500 735 600 — 832 800

M36 4 817 310 500 408 500 416 700 514 700 588 200 751 600 727 100 751 600 866 000 — 980 400

M39 4 976 370 900 488 000 497 800 614 900 702 700 897 900 868 600 897 900 1 035 000 — 1 171 000



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6 Proof load valuesProof load values are given in Table 6.The nominal stress area As is calculated as follows:

whered2

3) is the basic pitch diameter of the external thread;

d3 is the minor diameter of the external thread;

where

7 Failure loads for nuts with nominal height $ 0,5D but < 0,8D

The values of failure loads given in Table 7 for guidance apply to different bolt classes. Bolt stripping is the expected failure mode for lower strength bolts, while nut stripping can be expected for bolts of higher property classes. Table 7 — Minimum bolt stress when stripping

occurs

8 Test methods8.1 Proof load test

The proof load test shall be used wherever the capacity of available testing equipment permits, and shall be the referee method for sizes $ M5.The nut shall be assembled on a hardened and threaded test mandrel as shown in Figure 1 and Figure 2. For referee purposes, the axial tensile test is decisive.

The proof load shall be applied against the nut in an axial direction, and shall be held for 15 s. The nut shall resist the load without failure by stripping or rupture, and shall be removable by the fingers after the load is released. If the thread of the mandrel is damaged during the test, the test should be discarded. (It may be necessary to use a manual wrench to start the nut in motion. Such wrenching is permissible provided that it is restricted to one half turn and that the nut is then removable by the fingers.)The hardness of the test mandrel shall be 45 HRC minimum.Mandrels used shall be threaded to tolerance class 5h6g except that the tolerance of the major diameter shall be the last quarter of the 6g range on the minimum material side.

3) See ISO 724.

d1 is the basic minor diameter of the external thread;

H is the height of the fundamental triangle of the thread.

Property class of the

nut

Proof load stress of the

nut

Minimum stress in the core of bolt when stripping

occurs

N/mm2

N/mm2 for bolts with property class

6.8 8.8 10.9 12.9

04 380 260 300 330 350

05 500 290 370 410 480

Asπ4---

d2 d3+

2---------------------

2

=

d3 d1H6-----–=

Figure 1 — Axial tensile test



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EN 20898-2:1993

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8.2 Hardness test

For routine inspection, hardness tests shall be carried out on one bearing surface of the nut and the hardness shall be taken as the mean of three values spaced 120° apart. In case of dispute, the hardness tests shall be carried out on a longitudinal section through the nut axis and with impressions placed as close as possible to the nominal major diameter of the nut thread.The Vickers hardness test is the referee test, and where practicable a load of HV 30 shall be applied.If Brinell and Rockwell hardness tests are applied, the conversion tables in accordance with ISO 4964 shall be used.The Vickers hardness test shall be carried out in accordance with the requirements of ISO 6507-1.The Brinell hardness test shall be carried out in accordance with the requirements of ISO 6506.

The Rockwell hardness test shall be carried out in accordance with the requirements of ISO 6508.

8.3 Surface integrity test

For the surface integrity test, see ISO 6157-2.

9 Marking9.1 Symbols

Marking symbols are shown in Table 8 and Table 9.

9.2 Identification

Hexagon nuts of threads $ M5 and all property classes shall be marked in accordance with the designation system described in clause 3, by indenting on the side or bearing surface, or by embossing on the chamfer. See Figure 3 and Figure 4. Embossed marks shall not protrude beyond the bearing surface of the nut.

Figure 2 — Axial compressive test

Figure 3 — Examples of marking with designation symbol

Figure 4 — Examples of marking with code symbol (clock-face system)



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Table 8 — Marking symbols for nuts with property classes in accordance with 3.1

Table 9 — Marking for nuts with property classes in accordance with 3.2

Property class 4 5 6 8 9 10 12a

Alt

ern

ati

ve

ma

rkin

g either designation symbol 4 5 6 8 9 10 12

or code symbol (clock-face system)

a The marking dot cannot be replaced by the manufacturer’s mark.

Property class 04 05

Marking



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9.3 Marking of left-hand thread

Nuts with left-hand thread shall be marked as shown in Figure 5 on one bearing surface of the nut by indenting.

Marking is required for nuts with threads $ M5.The alternative marking for left-hand thread shown in Figure 6 may also be used.

9.4 Alternative marking

Alternative or optional permitted marking as stated in 9.1 to 9.3 is left to the choice of the manufacturer.

9.5 Trade (identification) marking

The trade (identification) marking of the manufacturer is mandatory on all products covered by the obligatory marking requirements for property classes, provided this is possible for technical reasons. Packages, however, shall be marked in all cases.

Figure 5 — Left-hand thread marking

Figure 6 — Alternative left-hand thread marking



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Annex A (informative) Loadability of bolted connections(Explanatory note concerning the specifications of Technical Committee ISO/TC 2 regarding nut strength and nut design.)Following the introduction of the ISO Recommendation on property classes for bolts and screws (ISO/R 898-1:1968), an ISO Recommendation on property classes for nuts(ISO/R 898-2) was published in 1969. These ISO Recommendations together produced a new system for the property classes of bolts, screws and nuts, and, in conjunction with new marking requirements, provided a clear statement of the loadability of a bolt-nut assembly.

a) In the case of bolts and screws, the symbol indicates:minimum tensile strength and yield to ultimate stress ratio.EXAMPLE

Properly class 8.8First figure (“8” in 8.8) = 1/100 of the minimum tensile strength, in newtons per square millimetre.Second figure (“8” in 8.8) = 10 times the yield stress ratio (0,8).Multiplication of these two figures (8 × 8 = 64) = 1/10 of the minimum yield stress, in newtons per square millimetre.

b) In the case of nuts:designation number = 1/100 of the minimum tensile strength, in newtons per square millimetre, of a bolt and screw, which, when mated with the nut, can be loaded up to the minimum yield stress.EXAMPLE

Bolt or screw 8.8 — nut 8connection loadable up to minimum yield stress of the bolt or screw.

Following publication of both ISO Recommendations, this system of property classes has been introduced worldwide and has proved to be a success.In 1973 the Sub-Committee SC 1 of ISO/TC 2 commenced revision of the ISO Recommendations on the basis of experience gathered and also planned to convert both Recommendations into ISO Standards. In 1974, a draft ISO/DIS 898-1 on property classes for bolts and screws was published, incorporating certain modifications and supplements, which, however, did not change the system of the property classes in principle. This draft was then revised once more. A second draft was prepared in 1977 and has since been adopted by a large majority of the member bodies of ISO. While considerable effort was required to develop thoroughly this draft concerning property classes for bolts and screws, it was finally resolved to the satisfaction of the interested countries within Sub-Committee SC 1 of ISO/TC 2 and now is agreed to by ISO. More extensive by far, and touching the substance of the specifications, was the work on a revised version of the ISO Recommendation ISO/R 898-2 and its conversion into an ISO Standard on property classes for nuts.Experience had shown that, while the concept of property classes in conjunction with a nominal 0,8D nut height is simple and straightforward, certain practical difficulties arise. First, it is sometimes difficult or impossible to achieve specified nut properties with the most economical materials and methods, for example with fine threads and certain sizes of coarse threads. Secondly, compliance with the requirements does not necessarily provide the assurance that the assembly would resist thread stripping during tightening. Previously it was considered adequate if the nut proof load was designed equal to the bolt minimum ultimate strength, however, the advent of yield point tightening methods and improved understanding of the interaction between nut and bolt threads showed the nuts required re-design to provide greater resistance to stripping of both the internal and external threads.



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For example, consider that the effective tensile strength of a bolt of class 8.8 may be between 800 N/mm2 and about 965 N/mm2 (determined from the maximum hardness) in sizes up to M16. Consequently the yield stress may range between 640 N/mm2 and 772 N/mm2 for a yield to ultimate stress ratio of 80 %. With the use of yield point tightening it will be seen that the tightening stress approaches the proof stress. Recent research has, in addition, shown that a nut tested with a hardened mandrel is capable or sustaining a higher load before stripping than when tested with a bolt of the appropriate property class. For example, a property class 8 nut when tested with a mandrel of 45 HRC will be capable of approximately 10 % higher load than when tested with a property class 8.8 bolt of dimensions similar to the mandrel. Therefore, a nut that just meets a proof stress of 800 N/mm2 with a hardened mandrel might only be expected to sustain a load of approximately 720 N/mm2 when mated with a property class 8.8 bolt of minimum dimensions. It will be seen that stripping of the threads may occur when tightening to stresses in excess of this, and from the bolt mechanical properties it will be seen that this could be a frequent occurrence with yield point tightening. It might be argued, however, that under torque tension loading the tensile strength of the bolt is reduced by about 15 %, but it should also be realized that the stripping strength of the assembly is also reduced by almost the same amount under torque tension loading. In addition to the introduction of yield point tightening methods, changes in certain ISO standards were under consideration that would also adversely affect this stripping tendency. Upgrading of bolt and screw mechanical properties was proposed as shown in Table A.1 (which is an excerpt from ISO 898-1), the purpose of which was to utilize fully the available strength of the commonly used materials for grades 4.8, 5.8, 8.8 (above M16), 10.9 and 12.9.

Another proposed change under consideration at this time was to reduce the width across flats of certain sizes of hexagon products to provide economies through optimized material use. As a result of these and other factors, certain member countries (Canada, Germany, Netherlands, Sweden, UK, USA) of Sub-Committee SC 1 of ISO/TC 2 conducted research and extensive testing of nut-bolt assemblies. Tests included a full variety of product sizes, strength levels and materials. In general, tests were conducted on typical production fasteners utilizing standard materials. Test parts were accurately measured for dimensions and material strength which then allowed appropriate statistical interpretation of the data. Results of the various investigators were evaluated by Canada and found to correlate well. A general series of formulae resulted that could be applied to predict the assembly strength of threaded components with the ISO 68 basic thread profile. These findings were thoroughly discussed within Sub-Committee SC 1 as well as in the various national committees.Despite the initial reluctance of the committee to permit changes in existing specifications, the test programme clearly indicated that there was inadequate resistance to assembly stripping, brought about largely by the improved tightening methods and upgrading of mechanical properties. The problem was both one of bolt thread stripping and nut thread stripping, and, as a result, it was concluded that the most viable means of overcoming the problem was by increasing the nominal 0,8D nut height where required. It is not the purpose of this annex to provide a detailed description of the tests conducted and the nut design method developed, for which the reader is referred to the following publication which provides a summary of results and the method employed: “Analysis and Design of Threaded Assemblies”, E.M. Alexander, 1977 SAE Transactions, Paper No. 770420.The calculation for nuts of property classes 4 to 6 according to the Alexander theory was not based on the maximum bolt hardness 250 HV, as given in ISO 898-1, see Table A.1, because this is a hardness which may occur at the bolt end or the head only. Therefore it was agreed to make calculations on the basis of the effective maximum hardnesses within the thread engagement part of the bolt, which are given in Table A.2.Similar graduated hardness values were specified in ISO/R 898-1:1968.



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EN 20898-2:1993

14 © BSI 01-1999

The above work showed that many factors influenced resistance of the stripping of threads, including tolerances, pitch, bell mouthing of nut minor diameter, size of countersink in nut, relative strength of nut threads to bolt threads, length of engagement, width across flats of nut, and style (for example hexagon flange), coefficient of friction, number of threads in the grip, etc. Analysis of the various sizes of fasteners on this basis indicated that it was not appropriate to have a fixed nominal nut height, for example 0,8D as before, but rather each standard assembly should be designed to give a suitable resistance to stripping. The result of this analysis gives the nut heights shown in Table A.3.It will be seen that there are two styles of nut, style 2 being approximately 10 % higher than style 1. Style 1 height is intended for property classes 4, 5, 6, 8, 10 and 12 (up to M16) in conjunction with appropriate mechanical properties, while style 2 dimensions are intended for use with property classes 8, 9 and 12, also with appropriate mechanical properties. The higher style of nut was primarily developed as an economical cold-formed nut to be used with property class 9.8 bolts and screws and it also provides suitable dimensions for a heat-treatable nut of good ductility for use with property class 12.9 bolts and screws. The intended applications of the two styles of nuts are detailed in Table 5, from which it is seen that this additional style of nut does not mean that dual stocking of part geometry will result.An overlapping between style 1 and style 2 occurs only in two cases. In the case of style 1, property class 8 allows the employment of nuts, not quenched and tempered (cold-worked low-carbon steel) only up to and including M16; above M16 the nut style 1 has to be quenched and tempered. However, it is possible in this case to use alternatively the thicker, not quenched and tempered, style 2. This is a question of economics in the final analysis. In the case of property class 12, it is not appropriate to use style 1 nuts above size M16. Due to the required proof loads, it would be necessary to raise the hardness of the nut to such an extent that its ductility, which is necessary from the functional point of view, would be impaired. Hence, the thicker style 2 nuts quenched and tempered are necessary in this case. If necessary, it would be possible to restrict the use of these nuts to sizes above M16, so that then no overlapping between style 1 and style 2 would occur in the case of property class 12.

Once nut dimensions were determined based on assembly strength criteria, the proof loads of these nuts with a restricted size hardened mandrel were determined. The result was that stresses under proof load were not constant for each property class of nut but varied with size. Accordingly, Table 5 shows revised stresses under proof load and hardness values for nuts. The property classes 04 and 05 (previously 06) for hexagon thin nuts with resultant reduced loadability are also indicated in this table. These nuts incidentally were not designed to provide resistance to stripping and are simply based on a fixed height of 0,6D.The stresses under proof load given in Table 5 are for the standard tolerance of 6H usually applied to nuts for mechanical fasteners. Where a larger tolerance or allowance is applied, these stresses should be modified by a factor as shown in Table 1.The values of Table 5 are only related to nuts with coarse thread. The same applies also to the test loads given in Table 1. For nuts with fine pitch thread, see ISO 898-6.The loads given in Table 1 are based on a test mandrel as specified in this part of ISO 898 with a minimum hardness of 45 HRC and thread tolerance of 5h6g (major diameter of 6g in the last quarter).ISO 898-1 and this part of ISO 898 on mechanical properties, ISO 4014 to ISO 4018 on hexagon bolts and screws, and ISO 4032 to ISO 4036 on hexagonal nuts have been published reflecting the revised mechanical properties, changes in nut heights and changes in width across flats (width across flats of M10, M12, M14 and M22 revised to 16 mm, 18 mm, 21 mm and 34 mm respectively from 17 mm, 19 mm, 22 mm and 32 mm) as recommended by ISO/TC 2.This part of ISO 898 makes the following statement concerning the property classes for nuts with full loadability:

A bolt or screw of a particular property class assembled with the equivalent property class of nut, in accordance with Table 2, is intended to provide an assembly capable of being tightened to achieve a bolt tension equivalent to the bolt proof load or yield load without stripping. Additionally, geometry and mechanical properties of nuts up to M39 and property class 12 of 6H thread tolerances are designed to provide for a high degree of resistance to stripping (at least 10 % bolt breakage of individual lots even under adverse minimum material conditions) when inadvertently overtorqued, in order to warn the user that the installation practice is not appropriate.



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EN 20898-2:1993

© BSI 01-1999 15

Certain users of the referenced standards could not, of necessity, participate in their detailed development and it is hoped that this explanatory note will provide increased understanding of this relatively complicated subject.

Table A.1 — Property classes for bolts and screws

Table A.2 — Effective maximum hardness within the thread engagement part of the bolt

Table A.3 — Heights of hexagon nuts

Property class3.6 4.6 4.8 5.6 5.8 6.8 8.8 9.8 10.9 12.9

# M16 > M16

Tensile strength, RmN/mm2

nom. 300 400 400 500 500 600 800 800 900 1 000 1 200

min. 330 400 420 500 520 600 800 830 900 1 040 1 220

Vickers hardness max. 250 HV 250 HV 250 HV 250 HV 250 HV 250 HV 320 HV 335 HV 360 HV 380 HV 435 HV

Property class Maximum hardness

3.6 158 HV

4.6; 4.8 180 HV

5.6; 5.8 220 HV

6.8 250 HV

ThreadWidth across flats

Nut height

Style 1 Style 2

min. max. m/D min. max. m/D

mm mm mm mm mm

M5 8 4,4 4,7 0,94 4,8 5,1 1,02

M6 10 4,9 5,2 0,87 5,4 5,7 0,95

M7 11 6,14 6,50 0,93 6,84 7,20 1,03

M8 13 6,44 6,80 0,85 7,14 7,50 0,94

M10 16 8,04 8,40 0,84 8,94 9,30 0,93

M12 18 10,37 10,80 0,90 11,57 12,00 1,00

M14 21 12,1 12,8 0,91 13,4 14,1 1,01

M16 24 14,1 14,8 0,92 15,7 16,4 1,02

M18 27 15,1 15,8 0,88 16,9 17,6 0,98

M20 30 16,9 18,0 0,90 19,0 20,3 1,02

M22 34 18,1 19,4 0,88 20,5 21,8 0,93

M24 36 20,2 21,5 0,90 22,6 23,9 1,00

M27 41 22,5 23,8 0,88 25,4 26,7 0,99

M30 46 24,3 25,6 0,85 27,3 28,6 0,95

M33 50 27,4 28,7 0,87 30,9 32,5 0,98

M36 55 29,4 31,0 0,86 33,1 34,7 0,96

M39 60 31,8 33,4 0,86 35,9 37,5 0,96



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EN 20898-2:1993

16 © BSI 01-1999

Annex B (informative) Bibliography[1] ISO 3506:1979, Corrosion-resistant stainless steel fasteners — Specifications.[2] ISO 4014:1988, Hexagon head bolts — Product grades A and B.[3] ISO 4015:1979, Hexagon head bolts — Product grade B — Reduced shank (shank diameter approximately equal to pitch diameter).[4] ISO 4016:1988, Hexagon head bolts — Product grade C.[5] ISO 4017:1988, Hexagon head screws — Product grades A and B.[6] ISO 4018:1988, Hexagon head screws — Product grade C.[7] ISO 4032:1986, Hexagon nuts, style 1 — Product grades A and B.[8] ISO 4033:1979, Hexagon nuts, style 2 — Product grades A and B.[9] ISO 4034:1986, Hexagon nuts — Product grade C.[10] ISO 4035:1986, Hexagon thin nuts (chamfered) — Product grades A and B.[11] ISO 4036:1979, Hexagon thin nuts — Product grade B (unchamfered).



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EN 20898-2:1993

© BSI 01-1999 17

Annex ZA (normative) Normative references to international publications with their relevant European publicationsThis European Standard incorporates by dated or undated reference, provisions from other publications. These normative references are cited at the appropriate places in the text and the publications are listed hereafter. For dated references, subsequent amendments to or revisions of any of these publications apply to this European Standard only when incorporated in it by amendment or revision. For undated references the latest edition of the publication referred to applies (including amendments). Publication Year Title EN/HD Year

ISO 1 1975 Standard reference temperature for industrial length measurements

— —

ISO 68 1973 ISO general purpose screw threads — Basic profile — —

ISO 261 1973 ISO general purpose metric screw threads — General plan — —

ISO 262 1973 ISO general purpose metric screw threads — Selected sizesfor screws, bolts and nuts

— —

ISO 272 1982 Fasteners — Hexagon products — Widths across flats — —

ISO 286-2 1988 ISO system of limits and fits — Part 2: Tables of standard tolerance grades and limit deviations for holes and shafts

EN 20286-2 1993

ISO 724 1978 ISO metric screw threads — Basic dimensions — —

ISO 965-1 1980 ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data

— —

ISO 965-2 1980 ISO general purpose metric screw threads — Tolerances — Part 2: Limits of sizes for general purpose bolt and nut threads — Medium quality

— —

ISO 4964 1984 Steel — Hardness conversions — —

ISO 6157-2 Fasteners — Surface discontinuities — Part 2: Nuts with threads M5 to M39

— —

ISO 6506 1981 Metallic materials — Hardness test — Brinell test — —

ISO 6507-1 1982 Metallic materials — Hardness test — Vickers test —Part 1: HV 5 to HV 100

— —

ISO 6508 1986 Metallic materials — Hardness test — Rockwell test(scales A – B – C – D – E – F – G – H – K)

— —



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18 blankCopyright British Standards Institution Provided by IHS under license with BSI


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BS EN 20898-2:1994

© BSI 01-1999

National annex NA (informative) Committees responsibleThe United Kingdom participation in the preparation of this European Standard was entrusted by the General Mechanical Engineering Standards Policy Committee (GME/-) to Technical Committee GME/9- upon which the following bodies were represented:

BEAMA Ltd.British Constructional Steelwork Association Ltd.British Industrial Fasteners FederationBritish Steel IndustryBritish Steel Industry (Wire Section)Gauge and Tool Makers’ AssociationMinistry of DefenceSociety of Motor Manufacturers and Traders Ltd.Washer Manufacturers’ Association of Great Britain

The following bodies were also represented in the drafting of the standard, through subcommittees and panels:

British Turned-parts Manufacturers’ AssociationInstitute of Metal FinishingMetal Finishing AssociationStainless Steel Fabricators’ Association of Great BritainNational Centre of Tribology

National annex NB (informative) Cross-references

Publication referred to Corresponding British Standard

ISO 68:1973 BS 3643 ISO metric screw threads

ISO 261:1973 Part 1:1981 Principles and basic data

ISO 262:1973

ISO 724:1978

ISO 965-1:1980

ISO 272:1982 BS 3692:1967 Specification for ISO metric precision hexagon bolts, screws and nuts. Metric units

BS 4190:1967 Specification for ISO metric black hexagon bolts, screws and nuts

BS 4395 Specification for high strength friction grip bolts and associated nuts and washers for structural engineering

Part 1:1969 General grade

Part 2:1969 Higher grade bolts and nuts and general grade washers

ISO 286-2:1988 BS 4500 ISO limits and fits

Section 1.2:1990 Tables of commonly used tolerance and grades and limits deviations for holes and shafts

ISO 6508:1986 BS 891:1989 Methods for hardness test (Rockwell method) and for verification of hardness testing machines (Rockwell method)



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